Method for masking multiple turbine components

ABSTRACT

Method to facilitate vapor phase coating of turbine airfoils. A dovetail receptacle receives the dovetails of a plurality of airfoils. The airfoils are separated by spacers with the airfoil dovetails sealed in a chamber of the receptacle to facilitate vapor phase coating of airfoil flowpath surfaces and platform upper surfaces of several airfoils simultaneously while masking the dovetail sections.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 09/545,542, filed Apr. 7,2000.

BACKGROUND OF THE INVENTION

This invention relates to coating gas turbine engine airfoils and, moreparticularly, to a method for masking airfoils during coating to preventdeposition of coating onto selected surfaces of such airfoils.

To provide protection against high temperatures and corrosive effects ofhot gases in gas turbine engines, turbine airfoils have been subjectedto aluminide coating. Aluminide coating on the flowpath surfaces,platform, angel wing and respective edges provides oxidation andcorrosion resistance and enhances the performance life of the airfoilcomponent. It is preferable, however, to avoid coating the dovetailsection below the platform of turbine airfoil components because coatingcan adversely affect fatigue life of the dovetail section. Furthermore,coating the dovetail section is not necessary because in service thissection is not in the hot gas flowpath, as is the airfoil section. It istherefore necessary to mask certain sections of the component to preventcoating such sections while allowing coating of other sections.

Two methods for coating turbine airfoils are a codeposition process anda vapor phase aluminide process. These are diffusion coating processeswhich use aluminum fluoride (AlF₃) gas to apply anoxidation-/corrosion-resistant, aluminum-rich layer to the componentsurface. The codeposition process involves sequential steps of degrease,tape and seal holes, wax, load box, slurry, seal, pack, run, unload andblowoff, water clean, and age. This process utilizes an activatedA1-rich powder packed tightly around the airfoil section to facilitatecoating while the areas below the platform are packed with an inertpowder containing calcined alumina and concentrations of Ni⁺² and Cr⁺³similar to the parent metal to avoid possible coating, as well as alloydepletion and inter-granular attack. The process yields an aluminidecoating that is brittle yet functional for prohibiting corrosion. Thisprocess is labor-intensive and operator-dependent, and tends to plugcooling holes and leave excessive amounts of foreign material on theairfoils. It also requires a final heat treatment, without which thecoating is too brittle.

The vapor phase aluminide process yields a ductile aluminide coatingwhich requires minimal final heat treatment. The vapor phase aluminideprocess does not require blow-off or water cleaning operations, thusreducing labor and material costs. One disadvantage of this process,however, is that coating must be removed from the dovetail sections bygrinding, and such grinding also removes the beneficial platform edgeand angel wing edge coating Thus, there is a need for an effectivemasking technique that facilitates secure masking of a plurality ofturbine airfoil components during vapor phase diffusion coatingprocesses.

SUMMARY OF THE INVENTION

Briefly, the invention is directed to a method for simultaneously vaporphase coating airfoil flowpath surfaces and upper platform surfaces of aplurality of turbine airfoils while masking dovetail sections of theturbine airfoils. The method involves inserting dovetail sections of aplurality of turbine airfoils into a dovetail section receptacle whichhas a chamber with an opening at the top of the receptacle for insertionof multiple turbine airfoil dovetail sections into the chamber, and arim around the opening, the rim being sized for supporting the platformsof the turbine airfoils, such that the dovetail section of each turbineairfoil extends into the chamber and the platform of each turbineairfoil rests on the rim, with upper platform surfaces and airfoilflowpath surfaces external to the receptacle. The method furtherinvolves inserting a spacer adjacent each platform of the turbineairfoils to prevent adjacent platforms from contacting each other,placing the dovetail section receptacle with dovetail sections andspacers therein into a coating apparatus, and introducing coating vaporinto the coating apparatus to form a coating on the airfoil flowpathsurfaces and upper platform surfaces external to the dovetail receptaclewhile avoiding coating of the dovetail sections inside the chamber.

The invention is also directed to a dovetail section receptacleapparatus for use in vapor phase diffusion coating of airfoil flowpathsurfaces and upper platform surfaces of a plurality of turbine airfoils,while avoiding coating of dovetail sections. The apparatus includes adovetail section receptacle for receiving dovetail sections of aplurality of turbine airfoils arranged side-by-side. The receptacle hasa chamber having an opening at the top of the receptacle for insertionof multiple turbine airfoil dovetail sections into the chamber, and arim around the opening, the rim being sized for supporting the platformsof the turbine airfoils side-by-side. The apparatus further includes aplurality of spacers for placement adjacent each platform of the turbineairfoils to prevent pairs of adjacent platforms from contacting eachother.

In another aspect the invention is directed to a turbine airfoil coatingassembly for vapor phase diffusion coating of airfoil flowpath surfacesand platform upper surfaces of turbine airfoils while avoiding coatingof dovetail sections thereof. The assembly includes a plurality ofturbine airfoils, each having a flowpath surface, a platform having aplatform upper surface, and a dovetail section. There is a dovetailsection receptacle for receiving dovetail sections of the plurality ofturbine airfoils arranged side-by-side, the dovetail section receptaclehaving a chamber with an opening having a perimeter at the top of thereceptacle for insertion of multiple turbine airfoil dovetail sectionsinto the chamber. There is a rim around the opening, the rim being sizedfor supporting the platforms of the turbine airfoils, such that thedovetail section of each turbine airfoil extends into the chamber andthe platform of each turbine airfoil rests on the rim, with upperplatform surfaces and flowpath surfaces external to the receptacle.There is a plurality of spacers, with a spacer adjacent each platform ofthe turbine airfoils to prevent adjacent platforms from contacting eachother.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are perspective views of airfoils;

FIG. 3 is a perspective view of a dovetail receptacle of the invention;

FIG. 4 is a side elevation of a spacer in accordance with the invention;

FIG. 5 is a left side elevation of the spacer of FIG. 4;

FIG. 6 is a right side elevation of the spacer of FIG. 4;

FIG. 7 is a side elevation of the spacer of FIG. 4 with a spacer barriercoating applied to a portion thereof; and

FIG. 8 is a perspective view of a turbine airfoil coating assemblyaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the process of the invention, a plurality of turbineairfoils such as airfoil 10 in FIG. 1 are placed within a specialreceptacle which facilitates coating of the airfoil flowpath surfaces12, platform upper surfaces 14, and platform edges 20 of the airfoilwhile preventing or at least minimizing coating dovetail surfaces of theturbine airfoil. The invention is also applicable to turbine airfoilshaving a stepped platform configuration as with turbine airfoil 30 shownin FIG. 2, and facilitates coating the airfoil flowpath surfaces,platform upper surfaces, platform edge surfaces 34, and platform wingextension edges 36. It is also applicable to other airfoils including,for example, stator vane assemblies where it may be desirable to coatairfoil flowpath surfaces as well as inner and outer bands of theassembly. This process utilizes an airfoil dovetail section receptacle60 (FIG. 3) which secures a plurality, preferably 10 or more, turbineairfoils during coating. The receptacle 60 forms a barrier whichseparates an internal volume 64 from an external volume 66. Thereceptacle is preferably rectangular and has a top surface 62, a frontsurface 68, a pair of side surfaces 70, a back surface 72, and a bottomsurface 74 cooperating to define a chamber, generally designated 76. Anopening at the top of the receptacle is sized and shaped to receive aseries of turbine airfoils. A rim 80 extends around the opening forsupporting airfoils when they are placed in the receptacle. In analternative embodiment, bottom surface 74 is eliminated and the floor ofthe coating apparatus (not shown) cooperates with the side, front andback surfaces of the receptacle to define the chamber.

The dovetail receptacle is constructed from a material capable ofwithstanding elevated vapor phase aluminiding temperatures on the orderof 1800° F.-2100° F. (980° C.-1150° C.). In the preferred embodiment,the dovetail receptacle is formed from purified graphite. Graphite isthe preferred material of construction because graphite is essentiallyinert to aluminide coating and does not accept aluminide coating whenexposed to aluminide vapor phase deposition processing.

Surfaces of the dovetail receptacle 60 are treated to avoidcarburization of the turbine platforms which seat against and contactthe rim 80 of the receptacle. In particular, the graphite surfaces ofthe receptacle which contact the airfoil are treated to avoid diffusionor other transfer of carbon from the receptacle to the airfoil. In thepreferred process, this is accomplished by applying a barrier layer torim 80 and optionally around a wall portion extending upward from therim 80. One preferred barrier layer is an aluminum oxide spray coatinglayer applied to these surfaces.

Another preferred barrier layer between the edges of the receptacle andthe platform edges of the airfoil is a strip of material applied to thereceptacle rim 80 which contacts the turbine airfoil platform and/orplatform wing extension (“angel wing”) edges. In one embodiment, this isa putty or tape strip which contains metallic and/or metallic oxidepowder. The strip itself acts as a physical barrier while the metallicor metallic oxide component powder serves a gettering function toattract coating vapor. The strip barrier function is accomplished, forexample, by M-1 or M-7 grade putty strip available from Chromallay ofIsrael under the trade designation “T-Block.” One advantage of thisstrip is that it is somewhat soft and therefore facilitates seating theturbine airfoil platform edges and/or angel wing edges therein byapplying light pressure. This barrier is disposed of after each coatingoperation to avoid release of gettered aluminide during subsequentcoating operations. The currently most preferred embodiment employs boththe aluminum oxide applied barrier layer and a strip overlay.

In assembling a plurality of turbine airfoils into the receptacle,spacers 90 illustrated in FIGS. 4-7 are placed between each turbineairfoil and abut against the side of each airfoil at or immediatelybelow each airfoil platform, as illustrated in FIG. 8. Inasmuch as it isdesirable for the edges of the platform to be exposed to aluminide vaporduring the subsequent vapor phase aluminide coating process, the spacersabut each adjacent platform so as to facilitate exposure to the vapor ofthe platform edges where they adjoin the platform upper surfaces whilepreventing vapor from traveling downward beneath the platform edges. Thespacers are shaped to account for the angel wings to similarlyfacilitate exposure to the vapor of the angel wing side edges whilepreventing vapor from traveling downward beneath the angel wing edges.For example, spacer 90 has an enlarged extension 94 which is sized andshaped to abut against the lateral edge of angel wing 36 shown in FIG.2. Spacer 90 also has an extension 96 which sized and shaped designed toabut against the extension of platform edge 34. The spacers somewhatloosely abut the platform and angel wing edges to in effect meter anamount of vapor therebetween for coating of the platform edges and angelwing edges while maintaining such metered volume of vapor sufficientlylow to prevent travel of appreciable vapor below such edges. Theportions of the spacers which are to abut the platform and angel wingedges of the airfoils shown at 92 in FIG. 4 are preferably coated withan aluminum oxide spray coating layer as illustrated at 92 in FIG. 7 toprevent transfer of carbon to the airfoil, similar to the manner inwhich certain surfaces of the receptacle are treated as described above.The spacer elements are preferably made from a material such as purifiedgraphite which is inert to the coating vapor and is not coated thereby.Graphite is also preferred because it contracts upon heating to coatingtemperatures, which effect cooperates with the expansion of the airfoilsupon heating, to maintain the desired relative abutment. Vapor from thevapor phase aluminide process during coating tends to enter coolingholes 18, 38 in the airfoil flowpath surface and travel down through theturbine airfoil. A quantity of this vapor traveling through interiorpassages (not shown) of the interior of the airfoil will then tend toexit the turbine airfoil at the base of the airfoil through openings 20,40 at the bottom of the dovetail, which openings serve as cooling gasinlets for the airfoil in service. If such vapor is permitted to exit,it will disadvantageously coat the dovetail section of the airfoilswhich the invention is designed to mask. To prevent this vapor fromexiting the turbine airfoil at the base of the dovetail, therefore, thisinvention employs a seal at the base as a barrier to this potential exitfor the vapor. This seal is preferably a strip of tape or putty of asimilar composition to the tape or putty described above for placementbetween the edges of the receptacle and the platform edges. This barrieris applied to each dovetail base prior to placement of each turbineairfoil in the receptacle. To assist in sealing this exit, it ispreferable that the depth of the receptacle be sized such that thebottom of the dovetail cooperates with the bottom of the receptacle tohold the strip in place.

Successive turbine airfoils separated by spacer elements are placed inthe receptacle to fill the receptacle as shown in FIG. 8 and form acomplete seal between the atmosphere above the platforms and below theplatforms. It is preferable that spacers also be positioned between thetwo outermost turbine airfoils and the ends of the receptacle.

After the turbine airfoils are assembled into the receptacle theairfoils are subjected to a conventional vapor phase aluminidingprocess. The components are then removed from the receptacle and brushedor water cleaned to remove residual masking media.

This process yields airfoil components having an aluminide coating onthe airfoil flowpath surfaces, platform upper surfaces, the platformedges, and angel wing edges where oxidation and corrosion resistance arerequired, and advantageously having no coating on dovetail sections.With this new masking technique, it is possible to coat components withthe vapor phase aluminide process which previously would be coated onlyby the codeposition process. Furthermore, the codeposition processdisadvantages such as plugged holes, external and internal foreignmaterials, coating brittleness, labor intensity, and slow productiontimes are avoided. The process of the invention is also superior toprior vapor phase aluminiding processes which involved grinding awayaluminide coating from certain surfaces because the stresses suchgrinding introduces to the component are avoided. And the processpermits the coating of a plurality of turbine airfoils simultaneouslywith a single dovetail receptacle. The new process therefore produces abetter product more efficiently than the prior processes.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:
 1. A method for simultaneously vapor phase coatingairfoil flowpath surfaces and upper platform surfaces of a plurality ofturbine airfoils while masking dovetail sections of said turbineairfoils, the method comprising: inserting dovetail sections of aplurality of turbine airfoils into a dovetail section receptaclecomprising a chamber having an opening at a top of the receptacle forinserting said plurality of turbine airfoil dovetail sections into thechamber, and a rim around the opening, the rim being sized forsupporting the platforms of the turbine airfoils, such that the dovetailsection of each turbine airfoil extends into said chamber and theplatform of each turbine airfoil rests on the rim, with upper platformsurfaces and airfoil flowpath surfaces external to the receptacle;inserting a spacer adjacent each platform of adjacent pairs of airfoilsof said plurality of turbine airfoils to prevent the platforms fromcontacting each other; placing the dovetail section receptacle withdovetail sections and spacers therein into a coating apparatus; andintroducing coating vapor into the coating apparatus to form a coatingon the airfoil flowpath surfaces and upper platform surfaces external tothe dovetail receptacle while avoiding coating the dovetail sectionsinside the chamber.
 2. The method of claim 1 wherein the spacers areconstructed from graphite and comprise a barrier coating to hinderdiffusion of carbon from the spacer into the turbine airfoil.
 3. Themethod of claim 1 wherein: a receptacle barrier coating comprisingaluminum oxide is applied to the rim to hinder diffusion of carbon fromthe rim into the turbine airfoils; a barrier strip is positioned to reston the rim between the rim and the platform of each turbine airfoil tohinder diffusion of carbon from the receptacle into the turbine airfoil,wherein the barrier strip comprises a gettering component within amatrix which is soft to facilitate seating of the platforms onto therim; and a spacer barrier coating comprising aluminum oxide is appliedto at least a portion of each spacer to hinder diffusion of carbon fromthe spacers into the turbine airfoils.
 4. The method of claim 1 whereinthe dovetail section receptacle is constructed from graphite andcomprises a barrier coating at said rim to hinder diffusion of carbonfrom the receptacle into the turbine airfoil.
 5. The method of claim 4wherein the spacers are constructed from graphite and comprise a barriercoating to hinder diffusion of carbon from the spacer into the turbineairfoil.
 6. The method of claim 4 wherein the barrier coating at saidrim comprises aluminum oxide.
 7. The method of claim 1 wherein eachturbine airfoil platform has a step thereon terminating in a platformwing extension and each spacer comprises a spacer extension which abutseach platform wing extension.
 8. The method of claim 7 wherein thespacers are constructed from graphite and comprise a spacer barriercoating to hinder diffusion of carbon from the spacers into the turbineairfoils.
 9. The method of claim 8 wherein the spacer barrier comprisesaluminum oxide.
 10. The method of claim 1 wherein the dovetail sectionreceptacle is constructed from graphite and a barrier strip ispositioned to rest on the rim between the rim and the platform of eachturbine airfoil to hinder diffusion of carbon from the receptacle intothe turbine airfoil.
 11. The method of claim 10 wherein the barrierstrip comprises a gettering component within a matrix which is soft tofacilitate seating of the platforms onto the rim.
 12. The method ofclaim 10 wherein the spacers abut the platform vertical edges in a loosemanner to permit vapor to contact said edges during coating.
 13. Themethod of claim 10 comprising a dovetail sealing strip applied to thedovetail section of each of said turbine airfoils to seal cooling gasinlets therein.
 14. A method for simultaneously vapor phase coatingairfoil flowpath surfaces and upper platform surfaces of a plurality ofturbine airfoils while masking dovetail sections of said turbineairfoils, the method comprising: inserting dovetail sections of aplurality of turbine airfoils into a dovetail section receptaclecomprising a chamber having an opening at a top of the receptacle forinserting said plurality of turbine airfoil dovetail sections into thechamber, and a rim around the opening, the rim being sized forsupporting the platforms of the turbine airfoils, such that the dovetailsection of each turbine airfoil extends into said chamber and theplatform of each turbine airfoil rests on the rim, with upper platformsurfaces and airfoil flowpath surfaces external to the receptacle;inserting a spacer adjacent each platform of adjacent pairs of airfoilsof said plurality of turbine airfoils to prevent the platforms fromcontacting each other; placing the dovetail section receptacle withdovetail sections and spacers therein into a coating apparatus; andintroducing coating vapor into the coating apparatus to form a coatingon the airfoil flowpath surfaces and upper platform surfaces external tothe dovetail receptacle while avoiding coating the dovetail sectionsinside the chamber; wherein the dovetail section receptacle isconstructed from graphite and comprises a barrier coating at said rim tohinder diffusion of carbon from the receptacle into the turbine airfoil;and wherein the spacers are constructed from graphite and comprise abarrier coating to hinder diffusion of carbon from the spacer into theturbine airfoil.
 15. A method for simultaneously vapor phase coatingairfoil flowpath surfaces and upper platform surfaces of a plurality ofturbine airfoils while masking dovetail sections of said turbineairfoils, the method comprising: inserting dovetail sections of aplurality of turbine airfoils into a dovetail section receptaclecomprising a chamber having an opening at a top of the receptacle forinserting said plurality of turbine airfoil dovetail sections into thechamber, and a rim around the opening, the rim being sized forsupporting the platforms of the turbine airfoils, such that the dovetailsection of each turbine airfoil extends into said chamber and theplatform of each turbine airfoil rests on the rim, with upper platformsurfaces and airfoil flowpath surfaces external to the receptacle;inserting a spacer adjacent each platform of adjacent pairs of airfoilsof said plurality of turbine airfoils to prevent the platforms fromcontacting each other; placing the dovetail section receptacle withdovetail sections and spacers therein into a coating apparatus; andintroducing coating vapor into the coating apparatus to form a coatingon the airfoil flowpath surfaces and upper platform surfaces external tothe dovetail receptacle while avoiding coating the dovetail sectionsinside the chamber; wherein the dovetail section receptacle isconstructed from graphite and a barrier strip is positioned to rest onthe rim between the rim and the platform of each turbine airfoil tohinder diffusion of carbon from the receptacle into the turbine airfoil;wherein the barrier strip comprises a gettering component within amatrix which is soft to facilitate seating of the platforms onto therim; wherein each turbine airfoil platform has a step thereonterminating in a platform wing extension and each spacer comprises aspacer extension which abuts each platform wing extension; wherein thespacers are constructed from graphite and comprise an aluminum oxidespacer barrier coating to hinder diffusion of carbon from the spacersinto the turbine airfoils.